1. Field of the Invention
This invention relates to an active three-axis controlled spacecraft.
2. Description of the Prior Art
Certain earth oriented satellites use a three-axis reaction wheel set for attitude control. Precision attitude control can be achieved by means of control torques developed by three reaction wheels in response to sensor-derived attitude error information. The three reaction wheels are classically mounted in the spacecraft in such a manner that their axes are parallel to an orthogonal set of body axes about which attitude control is to be maintained. For an earth-pointing spacecraft, one of the wheel axes is thus parallel to the spacecraft pitch axis which is nominally parallel to the orbit normal. The other two wheel axes are aligned with the spacecraft roll (velocity vector) and yaw (local vertical) axes, respectively. In the absence of large external disturbance torques, such a spacecraft system is referred to as a substantially zero-momentum system because the time average reaction wheel momentum about each axis is substantially zero.
Certain other earth oriented satellites use a single reaction wheel rotated about an axis oriented along the pitch axis of the spacecraft operating at an average non-zero, or bias, momentum so as to maintain the yaw axis of the main body portion of the spacecraft aligned to the local vertical while the inertial stability of the wheel maintains the pitch axis aligned to the orbit normal. such a satellite control system is a bias momentum controlled satellite and is embodied in the RCA SATCOM I and RCA SATCOM II satellites now in geosynchronous orbit operation, as well as several other satellites now in orbit. A significant feature of the bias momentum system is that the angular position of the body need be measured only about the roll and pitch axes, negating the more complex measurement of yaw.
In the zero momentum type of three axis stabilized attitude control for satellites, an attitude reference for each axis (i.e., roll, pitch, and yaw) is required to maintain the axes in the desired attitude and orientation. Whereas the earth itself (in the case of an earth orbiting satellite) provides a convenient reference for sensing pitch and roll errors as by use of a horizon sensor, other means involving the sun, certain stars, radio beacons and/or gyroscopes must be utilized to derive yaw axis information.
In order to avoid the use of either moving mechanical assemblies with high speed rotors, such as gyros, or of the operational complexities of star sensing, the sun can be selected as a convenient means to determine errors about the yaw axis for the non-eclipsed portion of the orbit. The sun is thus termed a yaw reference. However, in the vicinity of satellite noon or midnight, the sun cannot serve as a yaw reference, since the yaw axis approximately coincides with the sun line, so that decreased yaw sensitivity results. In fact, there can be no yaw signal at the point of coalignment. Operation of a zero momentum system with no yaw error input for these blind periods (eclipses) results in degraded pointing control. It is this defect or shortcoming in a zero momentum three-axis system depending on the sun for a yaw reference that detracts from the full use of a zero momentum system for a high accuracy attitude control where long life and reliability preclude other more complex yaw attitude devices.